(1) Field of the Invention
The invention relates to marine propulsion systems including drive shafts and propellers, and is directed more particularly to a marine propulsion system in which there is only a single drive shaft and two counter rotatable propellers.
(2) Description of the Prior Art
The use of counter-rotating propellers in marine propulsion systems is known. In particular, and referring to FIG. 1, such arrangements of propellers 10, 12 have found utility in torpedoes and unmanned undersea vehicles 14, and are favored in view of their efficiency, quiet operation, and the fact that they are torque balanced.
Known systems, as shown in FIG. 2, include two propeller drive shafts 16, 18, one for each propeller. Inasmuch as the two propellers 10, 12 typically are in axial alignment with each other, one of the drive shafts 18 is nested inside the other 16. A drive source 20, such as a motor or engine, drives the two shafts 16, 18. Bearings 22 are required between the two shafts, as well as between the outer shaft 16 and the vehicle body, or other support structure 24. Seals (not shown) are also required between the two shafts.
When an electric motor is used for the drive source 20, the use of two counter-rotating shafts requires a relatively complex motor, typically requiring a hole through the motor in which the inner shaft is disposed, or a secondary outer case which allows the field to rotate as well as the outer shaft.
There is a need for a simpler structure with respect to the drive shaft component of the assembly and a structure which permits use of an ordinary electric motor.